Characterization of Magnetic Bubble Domain Garnet Crystals by X-Ray Diffraction: Symmetry and Compositional Gradients

At room temperature, selected pieces of Gd2.34Tb0.66Fe5O12 are magnetically uniaxial and have excellent characteristics for bubble domain devices; but x-ray powder, precession, rotation, and Weissenberg photographs show no corresponding uniaxiality. Bond's method of accurate lattice spacing measurement shows d-spacing gradients which can be as large as 0.065% over a distance of 1.3 mm. These gradients are large along the normal to the natural (211) faces. The lattice parameters of the two end members of this solid-solution crystal were measured. Application of Vegard's law allows quantitative estimation of the compositional gradients due to varying rare-earth atom ratios. X-ray fluorescence analysis mapping showed no variation in rare-earth atom ratios. A small piece was selected as having less than 2 ppm gradient along [211]. This was used to determine the shape of the unit cell by measuring the d spacings of eight reflections (widely separated in reciprocal space). A least-squares refinement assuming rhombohedral symmetry and a [111] direction along the unique magnetic axis gave a rhombohedral angle of 90°−0.9 arc sec, σ=0.04 arc sec. This very small distortion could be due to compositional gradients remaining despite precautions and may not be inherent in a homogeneous crystal. The previous report of a 1/3-min distortion was the result of an unrecognized gradient.